T helper (Th) subsets are distinguished by their ability to produce distinct cytokine patterns and promote specific immune responses. Th1 cells produce IFNγ and promote cell-mediated immunity directed towards intracellular pathogens. In contrast, Th2 cells produce the cytokines IL-4, IL-5, and IL-13, activate mast cells and eosinophils and direct B cells against extracellular pathogens. Dysregulation of Th responses can result in immunopathology in that aberrant Th1 responses can be responsible for organ-specific autoimmunity, and exaggerated Th2 responses have been associated with allergic diseases.
The specific cytokines produced by polarized Th cells are the primary effectors that promote differentiation of precursor Th cells (Thp), but these cytokines also cross-regulate the other subset's functional activity. For example IL-4 is reported to be a potent factor in promoting the differentiation of Thp cells to Th2 effectors. In addition, IL-4 antagonizes production of IFNγ. IL-10, another cytokine produced by Th2 cells, has also been described to inhibit Th1 development and IFNγ-induced macrophage function. Conversely, the IFNγ produced by Th1 cells amplifies Th1 development and inhibits the expansion of Th2 cells. The ability of these cytokines to promote development of specific Th cell subsets, while simultaneously inhibiting the alternate developmental fate, results in a progressively polarized response.